Liquid crystal displays are commonly used as display devices for compact electronic apparatuses, because they not only are very thin but also provide good quality images with little power consumption. The liquid crystals in a liquid crystal display do not emit any light themselves. The liquid crystals have to be lit by a light source so as to clearly and sharply display text and images. Thus, a backlight module for an LCD is generally needed.
FIG. 6 shows a schematic, top view of a conventional LCD 300. The LCD 300 includes a flexible printed circuit board (FPC) 310, a liquid crystal display panel 320, a plastic frame 330, two light emitting diodes (LEDs) 340 cooperatively serving as a light source, and a light guide plate 400.
The FPC 310 connects with the liquid crystal display panel 320, and includes a main circuit area 311 and a light source setting area 312. The light source setting area 312 extends from the main circuit area 311. The LEDs 340 are arranged on the light source setting area 312 of the FPC 310. The frame 330 includes a depressed portion 331, and the depressed portion 331 has two openings 332.
The LCD 300 can be assembled according to the following sequence: firstly, setting the light guide plate 400 inside the frame 330; secondly, putting the light source setting area 312 into the depressed portion 331, and placing the LEDs 340 into the openings 332; thirdly, attaching the main circuit area 311 to the frame 330 and the light guide plate 400; and finally, attaching the liquid crystal display panel 320 to the frame 330.
However, the above-mentioned conventional liquid crystal display has the following problems.
The sizes of the openings 332 may not accurately match the sizes of the LEDs 340, due to imprecise manufacturing. When this happens, small gaps exist between the LEDs 340 and the light guide plate 400 after assembly. These gaps may diminish the emitting luminance of the light guide plate 400.
FIG. 7 is a graph showing the negative impact of the above mentioned gaps on the effective utilization of the emitting luminance. X represents the distance between the LEDs 340 and a light incident surface (not labeled) of the light guide plate 400 in millimeters (mm), and Y represents the relative emitting luminance of the light guide plate 400 as a function of X. It can be seen that the greater the value of X, the lower the value of Y That is, the greater the gap, the lower the value of the relative emitting luminance.
What is needed, therefore, is a backlight module and a liquid crystal display device using the same that overcome the above-described deficiencies.